This invention relates to communication receivers and, in particular, communication receivers for despreading multiple CDMA signals in the presence of multiple non-stationary narrow band interference.
Code Division Multiple Access (CDMA) Direct Sequence Spread Spectrum (DSSS) technologies promise to provide significant capacity improvements over analog and other digital communication systems. For these systems to achieve their promised capacity, they are required to adaptively control the power transmitted by each user within tight constraints. Unfortunately, this is not always possible, especially when the frequency band used is either adjacent to or shared by other narrow band systems such as analog or TDMA signals. Since these other users are not part of the CDMA system their power can not be controlled.
This problem is exacerbated by the non-stationary nature of these interfering signals. In a mobile environment rapid fluctuations in power are typically encountered due to the Raleigh and log-normal statistics of the channel. Furthermore, in a TDMA system each timeslot is used by a different user causing rapid fluctuations in received interference power at the TDMA burst rate. Finally, many mobile systems employ discontinuous transmission techniques so that they transmit only when the user is talking, thereby causing the interference to come and go with speech activity.
To operate in such an environment, several techniques may be used. First, all CDMA users can be commanded to transmit more power than would be necessary in the absence of the interference so that after despreading they have sufficient signal quality. This has the disadvantage of reducing the battery life of the portable users, and complicates system planning.
Secondly, adaptive interference techniques may be used. Classical techniques typically suffer from high complexity and often are unable to adapt quickly enough so that some interfering signal energy is still present. Further, these techniques are typically implemented as a front end processor separate from the despreader, and thus do not take full advantage of the properties of the signal.
Hence, what is needed, is a apparatus which does not add significant complexity to the CDMA receiver, but is able to instantaneously adapt to the presence of rapidly changing narrow band interference.